diff --git a/TODO b/TODO
index 5a2c205..45d0466 100644
--- a/TODO
+++ b/TODO
@@ -21,7 +21,6 @@ code:
 * bw_comb: integer target delay values?
 * prewarp control in all derived filtering modules
 * src inside distortions? w/ control from outside?
-* peak gain + Q ???
 * better src filter
 finally:
 * debugging - also check outputs different (incl bw_buf)
diff --git a/examples/fx_notch/vst3/Makefile b/examples/fx_notch/vst3/Makefile
index 7fb0f7e..59676a1 100644
--- a/examples/fx_notch/vst3/Makefile
+++ b/examples/fx_notch/vst3/Makefile
@@ -4,3 +4,6 @@ NAME := bw_example_fx_notch
 SOURCES = ${SOURCES_COMMON} ${ROOT_DIR}/../src/bw_example_fx_notch.c
 
 include ${ROOT_DIR}/../../common/vst3/vst3.mk
+
+CXXFLAGS += -DRELEASE=1 -DNDEBUG -DBW_NO_DEBUG
+#CXXFLAGS += -DDEVELOPMENT=1 -DBW_DEBUG_DEEP
diff --git a/examples/fxpp_notch/vst3/Makefile b/examples/fxpp_notch/vst3/Makefile
index 6348205..1f70060 100644
--- a/examples/fxpp_notch/vst3/Makefile
+++ b/examples/fxpp_notch/vst3/Makefile
@@ -4,3 +4,6 @@ NAME := bw_example_fxpp_notch
 SOURCES = ${SOURCES_COMMON} ${ROOT_DIR}/../src/bw_example_fxpp_notch.cpp
 
 include ${ROOT_DIR}/../../common/vst3/vst3.mk
+
+CXXFLAGS += -DRELEASE=1 -DNDEBUG -DBW_NO_DEBUG
+#CXXFLAGS += -DDEVELOPMENT=1 -DBW_DEBUG_DEEP
diff --git a/include/bw_notch.h b/include/bw_notch.h
index dabbad5..5219021 100644
--- a/include/bw_notch.h
+++ b/include/bw_notch.h
@@ -30,6 +30,12 @@
  *    <ul>
  *      <li>Version <strong>1.0.0</strong>:
  *        <ul>
+ *          <li>Added <code>bw_notch_reset_state_multi()</code> and updated C++
+ *              API in this regard.</li>
+ *          <li>Now <code>bw_notch_reset_state()</code> returns the initial
+ *              output value.</li>
+ *          <li>Added overloaded C++ <code>reset()</code> functions taking
+ *              arrays as arguments.</li>
  *          <li><code>bw_notch_process()</code> and
  *              <code>bw_notch_process_multi()</code> now use
  *              <code>size_t</code> to count samples and channels.</li>
@@ -39,6 +45,8 @@
  *          <li>Added overloaded C++ <code>process()</code> function taking
  *              C-style arrays as arguments.</li>
  *          <li>Removed usage of reserved identifiers.</li>
+ *          <li>Clearly specified parameter validity ranges.</li>
+ *          <li>Added debugging code.</li>
  *        </ul>
  *      </li>
  *      <li>Version <strong>0.6.0</strong>:
@@ -91,51 +99,86 @@ typedef struct bw_notch_state bw_notch_state;
  *
  *    #### bw_notch_init()
  *  ```>>> */
-static inline void bw_notch_init(bw_notch_coeffs *BW_RESTRICT coeffs);
+static inline void bw_notch_init(
+	bw_notch_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Initializes input parameter values in `coeffs`.
  *
  *    #### bw_notch_set_sample_rate()
  *  ```>>> */
-static inline void bw_notch_set_sample_rate(bw_notch_coeffs *BW_RESTRICT coeffs, float sample_rate);
+static inline void bw_notch_set_sample_rate(
+	bw_notch_coeffs * BW_RESTRICT coeffs,
+	float                         sample_rate);
 /*! <<<```
  *    Sets the `sample_rate` (Hz) value in `coeffs`.
  *
  *    #### bw_notch_reset_coeffs()
  *  ```>>> */
-static inline void bw_notch_reset_coeffs(bw_notch_coeffs *BW_RESTRICT coeffs);
+static inline void bw_notch_reset_coeffs(
+	bw_notch_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Resets coefficients in `coeffs` to assume their target values.
  *
  *    #### bw_notch_reset_state()
  *  ```>>> */
-static inline void bw_notch_reset_state(const bw_notch_coeffs *BW_RESTRICT coeffs, bw_notch_state *BW_RESTRICT state, float x_0);
+static inline float bw_notch_reset_state(
+	const bw_notch_coeffs * BW_RESTRICT coeffs,
+	bw_notch_state * BW_RESTRICT        state,
+	float                               x_0);
 /*! <<<```
  *    Resets the given `state` to its initial values using the given `coeffs`
  *    and the quiescent/initial input value `x_0`.
  *
+ *    Returns the corresponding quiescent/initial output value.
+ *
+ *    #### bw_notch_reset_state_multi()
+ *  ```>>> */
+static inline void bw_notch_reset_state_multi(
+	const bw_notch_coeffs * BW_RESTRICT              coeffs,
+	bw_notch_state * BW_RESTRICT const * BW_RESTRICT state,
+	const float *                                    x_0,
+	float *                                          y_0,
+	size_t                                           n_channels);
+/*! <<<```
+ *    Resets each of the `n_channels` `state`s to its initial values using the
+ *    given `coeffs` and the corresponding quiescent/initial input value in the
+ *    `x_0` array.
+ *
+ *    The corresponding quiescent/initial output values are written into the
+ *    `y_0` array, if not `NULL`.
+ *
  *    #### bw_notch_update_coeffs_ctrl()
  *  ```>>> */
-static inline void bw_notch_update_coeffs_ctrl(bw_notch_coeffs *BW_RESTRICT coeffs);
+static inline void bw_notch_update_coeffs_ctrl(
+	bw_notch_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers control-rate update of coefficients in `coeffs`.
  *
  *    #### bw_notch_update_coeffs_audio()
  *  ```>>> */
-static inline void bw_notch_update_coeffs_audio(bw_notch_coeffs *BW_RESTRICT coeffs);
+static inline void bw_notch_update_coeffs_audio(
+	bw_notch_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers audio-rate update of coefficients in `coeffs`.
  *
  *    #### bw_notch_process1()
  *  ```>>> */
-static inline float bw_notch_process1(const bw_notch_coeffs *BW_RESTRICT coeffs, bw_notch_state *BW_RESTRICT state, float x);
+static inline float bw_notch_process1(
+	const bw_notch_coeffs * BW_RESTRICT coeffs,
+	bw_notch_state * BW_RESTRICT        state,
+	float                               x);
 /*! <<<```
  *    Processes one input sample `x` using `coeffs`, while using and updating
  *    `state`. Returns the corresponding output sample.
  *
  *    #### bw_notch_process()
  *  ```>>> */
-static inline void bw_notch_process(bw_notch_coeffs *BW_RESTRICT coeffs, bw_notch_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples);
+static inline void bw_notch_process(
+	bw_notch_coeffs * BW_RESTRICT coeffs,
+	bw_notch_state * BW_RESTRICT  state,
+	const float *                 x,
+	float *                       y,
+	size_t                        n_samples);
 /*! <<<```
  *    Processes the first `n_samples` of the input buffer `x` and fills the
  *    first `n_samples` of the output buffer `y`, while using and updating both
@@ -143,7 +186,13 @@ static inline void bw_notch_process(bw_notch_coeffs *BW_RESTRICT coeffs, bw_notc
  *
  *    #### bw_notch_process_multi()
  *  ```>>> */
-static inline void bw_notch_process_multi(bw_notch_coeffs *BW_RESTRICT coeffs, bw_notch_state *BW_RESTRICT const *BW_RESTRICT state, const float * const *x, float * const *y, size_t n_channels, size_t n_samples);
+static inline void bw_notch_process_multi(
+	bw_notch_coeffs * BW_RESTRICT                    coeffs,
+	bw_notch_state * BW_RESTRICT const * BW_RESTRICT state,
+	const float * const *                            x,
+	float * const *                                  y,
+	size_t                                           n_channels,
+	size_t                                           n_samples);
 /*! <<<```
  *    Processes the first `n_samples` of the `n_channels` input buffers `x` and
  *    fills the first `n_samples` of the `n_channels` output buffers `y`, while
@@ -152,21 +201,81 @@ static inline void bw_notch_process_multi(bw_notch_coeffs *BW_RESTRICT coeffs, b
  *
  *    #### bw_notch_set_cutoff()
  *  ```>>> */
-static inline void bw_notch_set_cutoff(bw_notch_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_notch_set_cutoff(
+	bw_notch_coeffs * BW_RESTRICT coeffs,
+	float                       value);
 /*! <<<```
  *    Sets the cutoff frequency `value` (Hz) in `coeffs`.
  *
+ *    Valid range: [`1e-6f`, `1e12f`].
+ *
  *    Default value: `1e3f`.
  *
  *    #### bw_notch_set_Q()
  *  ```>>> */
-static inline void bw_notch_set_Q(bw_notch_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_notch_set_Q(
+	bw_notch_coeffs * BW_RESTRICT coeffs,
+	float                       value);
 /*! <<<```
  *    Sets the quality factor to the given `value` in `coeffs`.
  *
- *    `value` must be equal or bigger than `0.5f`.
+ *    Valid range: [`1e-6f`, `1e6f`].
  *
  *    Default value: `0.5f`.
+ *
+ *    #### bw_notch_set_prewarp_at_cutoff()
+ *  ```>>> */
+static inline void bw_notch_set_prewarp_at_cutoff(
+	bw_notch_coeffs * BW_RESTRICT coeffs,
+	char                        value);
+/*! <<<```
+ *    Sets whether bilinear transform prewarping frequency should match the
+ *    cutoff frequency (non-`0`) or not (`0`).
+ *
+ *    Default value: non-`0` (on).
+ *
+ *    #### bw_notch_set_prewarp_freq()
+ *  ```>>> */
+static inline void bw_notch_set_prewarp_freq(
+	bw_notch_coeffs * BW_RESTRICT coeffs,
+	float                       value);
+/*! <<<```
+ *    Sets the prewarping frequency `value` (Hz) in `coeffs`.
+ *
+ *    Only used when the prewarp\_at\_cutoff parameter is off and however
+ *    internally limited to avoid instability.
+ *
+ *    Valid range: [`1e-6f`, `1e12f`].
+ *
+ *    Default value: `1e3f`.
+ *
+ *    #### bw_notch_coeffs_is_valid()
+ *  ```>>> */
+static inline char bw_notch_coeffs_is_valid(
+	const bw_notch_coeffs * BW_RESTRICT coeffs);
+/*! <<<```
+ *    Tries to determine whether `coeffs` is valid and returns non-`0` if it
+ *    seems to be the case and `0` if it is certainly not. False positives are
+ *    possible, false negatives are not.
+ *
+ *    `coeffs` must at least point to a readable memory block of size greater
+ *    than or equal to that of `bw_notch_coeffs`.
+ *
+ *    #### bw_notch_state_is_valid()
+ *  ```>>> */
+static inline char bw_notch_state_is_valid(
+	const bw_notch_coeffs * BW_RESTRICT coeffs,
+	const bw_notch_state * BW_RESTRICT  state);
+/*! <<<```
+ *    Tries to determine whether `state` is valid and returns non-`0` if it
+ *    seems to be the case and `0` if it is certainly not. False positives are
+ *    possible, false negatives are not.
+ *
+ *    If `coeffs` is not `NULL` extra cross-checks might be performed (`state`
+ *    is supposed to be associated to `coeffs`).
+ *
+ *    `state` must at least point to a readable memory block of size greater
+ *    than or equal to that of `bw_notch_state`.
  *  }}} */
 
 #ifdef __cplusplus
@@ -184,68 +293,320 @@ static inline void bw_notch_set_Q(bw_notch_coeffs *BW_RESTRICT coeffs, float val
 extern "C" {
 #endif
 
+#ifdef BW_DEBUG_DEEP
+enum bw_notch_coeffs_state {
+	bw_notch_coeffs_state_invalid,
+	bw_notch_coeffs_state_init,
+	bw_notch_coeffs_state_set_sample_rate,
+	bw_notch_coeffs_state_reset_coeffs
+};
+#endif
+
 struct bw_notch_coeffs {
+#ifdef BW_DEBUG_DEEP
+	uint32_t			hash;
+	enum bw_notch_coeffs_state	state;
+	uint32_t			reset_id;
+#endif
+
 	// Sub-components
-	bw_svf_coeffs	svf_coeffs;
+	bw_svf_coeffs			svf_coeffs;
 };
 
 struct bw_notch_state {
+#ifdef BW_DEBUG_DEEP
+	uint32_t	hash;
+	uint32_t	coeffs_reset_id;
+#endif
+
 	bw_svf_state	svf_state;
 };
 
-static inline void bw_notch_init(bw_notch_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_notch_init(
+		bw_notch_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
 	bw_svf_init(&coeffs->svf_coeffs);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->hash = bw_hash_sdbm("bw_notch_coeffs");
+	coeffs->state = bw_notch_coeffs_state_init;
+	coeffs->reset_id = coeffs->hash + 1;
+#endif
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_notch_coeffs_state_init);
 }
 
-static inline void bw_notch_set_sample_rate(bw_notch_coeffs *BW_RESTRICT coeffs, float sample_rate) {
+static inline void bw_notch_set_sample_rate(
+		bw_notch_coeffs * BW_RESTRICT coeffs,
+		float                         sample_rate) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(sample_rate) && sample_rate > 0.f);
+
 	bw_svf_set_sample_rate(&coeffs->svf_coeffs, sample_rate);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_notch_coeffs_state_set_sample_rate;
+#endif
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_notch_coeffs_state_set_sample_rate);
 }
 
-static inline void bw_notch_reset_coeffs(bw_notch_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_notch_reset_coeffs(
+		bw_notch_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_set_sample_rate);
+
 	bw_svf_reset_coeffs(&coeffs->svf_coeffs);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_notch_coeffs_state_reset_coeffs;
+	coeffs->reset_id++;
+#endif
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_notch_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_notch_reset_state(const bw_notch_coeffs *BW_RESTRICT coeffs, bw_notch_state *BW_RESTRICT state, float x_0) {
-	bw_svf_reset_state(&coeffs->svf_coeffs, &state->svf_state, x_0);
+static inline float bw_notch_reset_state(
+		const bw_notch_coeffs * BW_RESTRICT coeffs,
+		bw_notch_state * BW_RESTRICT        state,
+		float                               x_0) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(bw_is_finite(x_0));
+
+	float lp, bp, hp;
+	bw_svf_reset_state(&coeffs->svf_coeffs, &state->svf_state, x_0, &lp, &bp, &hp);
+	const float y = x_0;
+
+#ifdef BW_DEBUG_DEEP
+	state->hash = bw_hash_sdbm("bw_notch_state");
+	state->coeffs_reset_id = coeffs->reset_id;
+#endif
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_notch_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(y));
+
+	return y;
 }
 
-static inline void bw_notch_update_coeffs_ctrl(bw_notch_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_notch_reset_state_multi(
+		const bw_notch_coeffs * BW_RESTRICT              coeffs,
+		bw_notch_state * BW_RESTRICT const * BW_RESTRICT state,
+		const float *                                    x_0,
+		float *                                          y_0,
+		size_t                                           n_channels) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(x_0 != NULL);
+
+	if (y_0 != NULL)
+		for (size_t i = 0; i < n_channels; i++)
+			y_0[i] = bw_notch_reset_state(coeffs, state[i], x_0[i]);
+	else
+		for (size_t i = 0; i < n_channels; i++)
+			bw_notch_reset_state(coeffs, state[i], x_0[i]);
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(y_0 != NULL ? bw_has_only_finite(y_0, n_channels) : 1);
+}
+
+static inline void bw_notch_update_coeffs_ctrl(
+		bw_notch_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+
 	bw_svf_update_coeffs_ctrl(&coeffs->svf_coeffs);
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_notch_update_coeffs_audio(bw_notch_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_notch_update_coeffs_audio(
+		bw_notch_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+
 	bw_svf_update_coeffs_audio(&coeffs->svf_coeffs);
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
 }
 
-static inline float bw_notch_process1(const bw_notch_coeffs *BW_RESTRICT coeffs, bw_notch_state *BW_RESTRICT state, float x) {
+static inline float bw_notch_process1(
+		const bw_notch_coeffs * BW_RESTRICT coeffs,
+		bw_notch_state * BW_RESTRICT        state,
+		float                               x) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_notch_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(x));
+
 	float lp, bp, hp;
 	bw_svf_process1(&coeffs->svf_coeffs, &state->svf_state, x, &lp, &bp, &hp);
-	return lp + hp;
+	const float y = lp + hp;
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_notch_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(y));
+
+	return y;
 }
 
-static inline void bw_notch_process(bw_notch_coeffs *BW_RESTRICT coeffs, bw_notch_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples) {
+static inline void bw_notch_process(
+		bw_notch_coeffs * BW_RESTRICT coeffs,
+		bw_notch_state * BW_RESTRICT  state,
+		const float *                 x,
+		float *                       y,
+		size_t                        n_samples) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_notch_state_is_valid(coeffs, state));
+	BW_ASSERT(x != NULL);
+	BW_ASSERT_DEEP(bw_has_only_finite(x, n_samples));
+	BW_ASSERT(y != NULL);
+
 	bw_notch_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_notch_update_coeffs_audio(coeffs);
 		y[i] = bw_notch_process1(coeffs, state, x[i]);
 	}
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_notch_state_is_valid(coeffs, state));
+	BW_ASSERT_DEEP(bw_has_only_finite(y, n_samples));
 }
 
-static inline void bw_notch_process_multi(bw_notch_coeffs *BW_RESTRICT coeffs, bw_notch_state *BW_RESTRICT const *BW_RESTRICT state, const float * const *x, float * const *y, size_t n_channels, size_t n_samples) {
+static inline void bw_notch_process_multi(
+		bw_notch_coeffs * BW_RESTRICT                    coeffs,
+		bw_notch_state * BW_RESTRICT const * BW_RESTRICT state,
+		const float * const *                            x,
+		float * const *                                  y,
+		size_t                                           n_channels,
+		size_t                                           n_samples) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(x != NULL);
+	BW_ASSERT(y != NULL);
+
 	bw_notch_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_notch_update_coeffs_audio(coeffs);
 		for (size_t j = 0; j < n_channels; j++)
 			y[j][i] = bw_notch_process1(coeffs, state[j], x[j][i]);
 	}
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_notch_set_cutoff(bw_notch_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_notch_set_cutoff(
+		bw_notch_coeffs * BW_RESTRICT coeffs,
+		float                         value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value >= 1e-6f && value <= 1e12f);
+
 	bw_svf_set_cutoff(&coeffs->svf_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_init);
 }
 
-static inline void bw_notch_set_Q(bw_notch_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_notch_set_Q(
+		bw_notch_coeffs * BW_RESTRICT coeffs,
+		float                         value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value >= 1e-6f && value <= 1e6f);
+
 	bw_svf_set_Q(&coeffs->svf_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_init);
+}
+
+static inline void bw_notch_set_prewarp_at_cutoff(
+		bw_notch_coeffs * BW_RESTRICT coeffs,
+		char                          value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_init);
+
+	bw_svf_set_prewarp_at_cutoff(&coeffs->svf_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_init);
+}
+
+static inline void bw_notch_set_prewarp_freq(
+		bw_notch_coeffs * BW_RESTRICT coeffs,
+		float                         value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value >= 1e-6f && value <= 1e12f);
+
+	bw_svf_set_prewarp_freq(&coeffs->svf_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_notch_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_notch_coeffs_state_init);
+}
+
+static inline char bw_notch_coeffs_is_valid(
+		const bw_notch_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (coeffs->hash != bw_hash_sdbm("bw_notch_coeffs"))
+		return 0;
+	if (coeffs->state < bw_notch_coeffs_state_init || coeffs->state > bw_notch_coeffs_state_reset_coeffs)
+		return 0;
+#endif
+
+	return bw_svf_coeffs_is_valid(&coeffs->svf_coeffs);
+}
+
+static inline char bw_notch_state_is_valid(
+		const bw_notch_coeffs * BW_RESTRICT coeffs,
+		const bw_notch_state * BW_RESTRICT  state) {
+	BW_ASSERT(state != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (state->hash != bw_hash_sdbm("bw_notch_state"))
+		return 0;
+
+	if (coeffs != NULL && coeffs->reset_id != state->coeffs_reset_id)
+		return 0;
+#endif
+
+	(void)coeffs;
+
+	return bw_svf_state_is_valid(&coeffs->svf_coeffs, &state->svf_state);
 }
 
 #ifdef __cplusplus
@@ -265,19 +626,46 @@ class Notch {
 public:
 	Notch();
 
-	void setSampleRate(float sampleRate);
-	void reset(float x_0 = 0.f);
+	void setSampleRate(
+		float sampleRate);
+
+	void reset(
+		float               x0 = 0.f,
+		float * BW_RESTRICT y0 = nullptr);
+
+	void reset(
+		float                                       x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0);
+
+	void reset(
+		const float * x0,
+		float *       y0 = nullptr);
+
+	void reset(
+		std::array<float, N_CHANNELS>               x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0 = nullptr);
+
 	void process(
-		const float * const *x,
-		float * const *y,
-		size_t nSamples);
+		const float * const * x,
+		float * const *       y,
+		size_t                nSamples);
+
 	void process(
 		std::array<const float *, N_CHANNELS> x,
-		std::array<float *, N_CHANNELS> y,
-		size_t nSamples);
+		std::array<float *, N_CHANNELS>       y,
+		size_t                                nSamples);
 
-	void setCutoff(float value);
-	void setQ(float value);
+	void setCutoff(
+		float value);
+
+	void setQ(
+		float value);
+
+	void setPrewarpAtCutoff(
+		bool value);
+
+	void setPrewarpFreq(
+		float value);
 /*! <<<...
  *  }
  *  ```
@@ -289,9 +677,9 @@ public:
  * change at any time in future versions. Please, do not use it directly. */
 
 private:
-	bw_notch_coeffs	 coeffs;
-	bw_notch_state	 states[N_CHANNELS];
-	bw_notch_state	*BW_RESTRICT statesP[N_CHANNELS];
+	bw_notch_coeffs			coeffs;
+	bw_notch_state			states[N_CHANNELS];
+	bw_notch_state * BW_RESTRICT	statesP[N_CHANNELS];
 };
 
 template<size_t N_CHANNELS>
@@ -302,43 +690,86 @@ inline Notch<N_CHANNELS>::Notch() {
 }
 
 template<size_t N_CHANNELS>
-inline void Notch<N_CHANNELS>::setSampleRate(float sampleRate) {
+inline void Notch<N_CHANNELS>::setSampleRate(
+		float sampleRate) {
 	bw_notch_set_sample_rate(&coeffs, sampleRate);
 }
 
 template<size_t N_CHANNELS>
-inline void Notch<N_CHANNELS>::reset(float x_0) {
+inline void Notch<N_CHANNELS>::reset(
+		float               x0,
+		float * BW_RESTRICT y0) {
 	bw_notch_reset_coeffs(&coeffs);
-	for (size_t i = 0; i < N_CHANNELS; i++)
-		bw_notch_reset_state(&coeffs, states + i, x_0);
+	if (y0 != nullptr)
+		for (size_t i = 0; i < N_CHANNELS; i++)
+			y0[i] = bw_notch_reset_state(&coeffs, states + i, x0);
+	else
+		for (size_t i = 0; i < N_CHANNELS; i++)
+			bw_notch_reset_state(&coeffs, states + i, x0);
+}
+
+template<size_t N_CHANNELS>
+inline void Notch<N_CHANNELS>::reset(
+		float                                       x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0) {
+	reset(x0, y0 != nullptr ? y0->data() : nullptr);
+}
+
+template<size_t N_CHANNELS>
+inline void Notch<N_CHANNELS>::reset(
+		const float * x0,
+		float *       y0) {
+	bw_notch_reset_coeffs(&coeffs);
+	bw_notch_reset_state_multi(&coeffs, statesP, x0, y0, N_CHANNELS);
+}
+
+template<size_t N_CHANNELS>
+inline void Notch<N_CHANNELS>::reset(
+		std::array<float, N_CHANNELS>               x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0) {
+	reset(x0.data(), y0 != nullptr ? y0->data() : nullptr);
 }
 
 template<size_t N_CHANNELS>
 inline void Notch<N_CHANNELS>::process(
-		const float * const *x,
-		float * const *y,
-		size_t nSamples) {
+		const float * const * x,
+		float * const *       y,
+		size_t                nSamples) {
 	bw_notch_process_multi(&coeffs, statesP, x, y, N_CHANNELS, nSamples);
 }
 
 template<size_t N_CHANNELS>
 inline void Notch<N_CHANNELS>::process(
 		std::array<const float *, N_CHANNELS> x,
-		std::array<float *, N_CHANNELS> y,
-		size_t nSamples) {
+		std::array<float *, N_CHANNELS>       y,
+		size_t                                nSamples) {
 	process(x.data(), y.data(), nSamples);
 }
 
 template<size_t N_CHANNELS>
-inline void Notch<N_CHANNELS>::setCutoff(float value) {
+inline void Notch<N_CHANNELS>::setCutoff(
+		float value) {
 	bw_notch_set_cutoff(&coeffs, value);
 }
 
 template<size_t N_CHANNELS>
-inline void Notch<N_CHANNELS>::setQ(float value) {
+inline void Notch<N_CHANNELS>::setQ(
+		float value) {
 	bw_notch_set_Q(&coeffs, value);
 }
 
+template<size_t N_CHANNELS>
+inline void Notch<N_CHANNELS>::setPrewarpAtCutoff(
+		bool value) {
+	bw_notch_set_prewarp_at_cutoff(&coeffs, value);
+}
+
+template<size_t N_CHANNELS>
+inline void Notch<N_CHANNELS>::setPrewarpFreq(
+		float value) {
+	bw_notch_set_prewarp_freq(&coeffs, value);
+}
+
 }
 #endif
 
diff --git a/include/bw_peak.h b/include/bw_peak.h
index d52b228..6738b67 100644
--- a/include/bw_peak.h
+++ b/include/bw_peak.h
@@ -37,6 +37,12 @@
  *    <ul>
  *      <li>Version <strong>1.0.0</strong>:
  *        <ul>
+ *          <li>Added <code>bw_peak_reset_state_multi()</code> and updated C++
+ *              API in this regard.</li>
+ *          <li>Now <code>bw_peak_reset_state()</code> returns the initial
+ *              output value.</li>
+ *          <li>Added overloaded C++ <code>reset()</code> functions taking
+ *              arrays as arguments.</li>
  *          <li><code>bw_peak_process()</code> and
  *              <code>bw_peak_process_multi()</code> now use <code>size_t</code>
  *              to count samples and channels.</li>
@@ -48,6 +54,8 @@
  *          <li>Removed usage of reserved identifiers.</li>
  *          <li>Added pragmas to silence bogus GCC uninitialized variable
  *              warnings.</li>
+ *          <li>Clearly specified parameter validity ranges.</li>
+ *          <li>Added debugging code.</li>
  *        </ul>
  *      </li>
  *      <li>Version <strong>0.6.0</strong>:
@@ -102,51 +110,86 @@ typedef struct bw_peak_state bw_peak_state;
  *
  *    #### bw_peak_init()
  *  ```>>> */
-static inline void bw_peak_init(bw_peak_coeffs *BW_RESTRICT coeffs);
+static inline void bw_peak_init(
+	bw_peak_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Initializes input parameter values in `coeffs`.
  *
  *    #### bw_peak_set_sample_rate()
  *  ```>>> */
-static inline void bw_peak_set_sample_rate(bw_peak_coeffs *BW_RESTRICT coeffs, float sample_rate);
+static inline void bw_peak_set_sample_rate(
+	bw_peak_coeffs *BW_RESTRICT coeffs,
+	float                       sample_rate);
 /*! <<<```
  *    Sets the `sample_rate` (Hz) value in `coeffs`.
  *
  *    #### bw_peak_reset_coeffs()
  *  ```>>> */
-static inline void bw_peak_reset_coeffs(bw_peak_coeffs *BW_RESTRICT coeffs);
+static inline void bw_peak_reset_coeffs(
+	bw_peak_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Resets coefficients in `coeffs` to assume their target values.
  *
  *    #### bw_peak_reset_state()
  *  ```>>> */
-static inline void bw_peak_reset_state(const bw_peak_coeffs *BW_RESTRICT coeffs, bw_peak_state *BW_RESTRICT state, float x_0);
+static inline float bw_peak_reset_state(
+	const bw_peak_coeffs * BW_RESTRICT coeffs,
+	bw_peak_state * BW_RESTRICT        state,
+	float                              x_0);
 /*! <<<```
  *    Resets the given `state` to its initial values using the given `coeffs`
  *    and the quiescent/initial input value `x_0`.
  *
+ *    Returns the corresponding quiescent/initial output value.
+ *
+ *    #### bw_peak_reset_state_multi()
+ *  ```>>> */
+static inline void bw_peak_reset_state_multi(
+	const bw_peak_coeffs * BW_RESTRICT              coeffs,
+	bw_peak_state * BW_RESTRICT const * BW_RESTRICT state,
+	const float *                                   x_0,
+	float *                                         y_0,
+	size_t                                          n_channels);
+/*! <<<```
+ *    Resets each of the `n_channels` `state`s to its initial values using the
+ *    given `coeffs` and the corresponding quiescent/initial input value in the
+ *    `x_0` array.
+ *
+ *    The corresponding quiescent/initial output values are written into the
+ *    `y_0` array, if not `NULL`.
+ *
  *    #### bw_peak_update_coeffs_ctrl()
  *  ```>>> */
-static inline void bw_peak_update_coeffs_ctrl(bw_peak_coeffs *BW_RESTRICT coeffs);
+static inline void bw_peak_update_coeffs_ctrl(
+	bw_peak_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers control-rate update of coefficients in `coeffs`.
  *
  *    #### bw_peak_update_coeffs_audio()
  *  ```>>> */
-static inline void bw_peak_update_coeffs_audio(bw_peak_coeffs *BW_RESTRICT coeffs);
+static inline void bw_peak_update_coeffs_audio(
+	bw_peak_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers audio-rate update of coefficients in `coeffs`.
  *
  *    #### bw_peak_process1()
  *  ```>>> */
-static inline float bw_peak_process1(const bw_peak_coeffs *BW_RESTRICT coeffs, bw_peak_state *BW_RESTRICT state, float x);
+static inline float bw_peak_process1(
+	const bw_peak_coeffs * BW_RESTRICT coeffs,
+	bw_peak_state * BW_RESTRICT        state,
+	float                              x);
 /*! <<<```
  *    Processes one input sample `x` using `coeffs`, while using and updating
  *    `state`. Returns the corresponding output sample.
  *
  *    #### bw_peak_process()
  *  ```>>> */
-static inline void bw_peak_process(bw_peak_coeffs *BW_RESTRICT coeffs, bw_peak_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples);
+static inline void bw_peak_process(
+	bw_peak_coeffs * BW_RESTRICT coeffs,
+	bw_peak_state * BW_RESTRICT  state,
+	const float *                x,
+	float *                      y,
+	size_t                       n_samples);
 /*! <<<```
  *    Processes the first `n_samples` of the input buffer `x` and fills the
  *    first `n_samples` of the output buffer `y`, while using and updating both
@@ -154,7 +197,13 @@ static inline void bw_peak_process(bw_peak_coeffs *BW_RESTRICT coeffs, bw_peak_s
  *
  *    #### bw_peak_process_multi()
  *  ```>>> */
-static inline void bw_peak_process_multi(bw_peak_coeffs *BW_RESTRICT coeffs, bw_peak_state *BW_RESTRICT const *BW_RESTRICT state, const float * const *x, float * const *y, size_t n_channels, size_t n_samples);
+static inline void bw_peak_process_multi(
+	bw_peak_coeffs * BW_RESTRICT                    coeffs,
+	bw_peak_state * BW_RESTRICT const * BW_RESTRICT state,
+	const float * const *                           x,
+	float * const *                                 y,
+	size_t                                          n_channels,
+	size_t                                          n_samples);
 /*! <<<```
  *    Processes the first `n_samples` of the `n_channels` input buffers `x` and
  *    fills the first `n_samples` of the `n_channels` output buffers `y`, while
@@ -163,55 +212,147 @@ static inline void bw_peak_process_multi(bw_peak_coeffs *BW_RESTRICT coeffs, bw_
  *
  *    #### bw_peak_set_cutoff()
  *  ```>>> */
-static inline void bw_peak_set_cutoff(bw_peak_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_peak_set_cutoff(
+	bw_peak_coeffs * BW_RESTRICT coeffs,
+	float                        value);
 /*! <<<```
  *    Sets the cutoff frequency `value` (Hz) in `coeffs`.
  *
+ *    Valid range: [`1e-6f`, `1e12f`].
+ *
  *    Default value: `1e3f`.
  *
  *    #### bw_peak_set_Q()
  *  ```>>> */
-static inline void bw_peak_set_Q(bw_peak_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_peak_set_Q(
+	bw_peak_coeffs * BW_RESTRICT coeffs,
+	float                        value);
 /*! <<<```
  *    Sets the quality factor to the given `value` in `coeffs`.
  *
- *    `value` must be equal or bigger than `0.5f`.
+ *    Valid range: [`1e-6f`, `1e6f`].
  *
  *    Default value: `0.5f`.
  *
+ *    #### bw_peak_set_prewarp_at_cutoff()
+ *  ```>>> */
+static inline void bw_peak_set_prewarp_at_cutoff(
+	bw_peak_coeffs * BW_RESTRICT coeffs,
+	char                         value);
+/*! <<<```
+ *    Sets whether bilinear transform prewarping frequency should match the
+ *    cutoff frequency (non-`0`) or not (`0`).
+ *
+ *    Default value: non-`0` (on).
+ *
+ *    #### bw_peak_set_prewarp_freq()
+ *  ```>>> */
+static inline void bw_peak_set_prewarp_freq(
+	bw_peak_coeffs * BW_RESTRICT coeffs,
+	float                        value);
+/*! <<<```
+ *    Sets the prewarping frequency `value` (Hz) in `coeffs`.
+ *
+ *    Only used when the prewarp\_at\_cutoff parameter is off and however
+ *    internally limited to avoid instability.
+ *
+ *    Valid range: [`1e-6f`, `1e12f`].
+ *
+ *    Default value: `1e3f`.
+ *
  *    #### bw_peak_set_peak_gain_lin()
  *  ```>>> */
-static inline void bw_peak_set_peak_gain_lin(bw_peak_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_peak_set_peak_gain_lin(
+	bw_peak_coeffs * BW_RESTRICT coeffs,
+	float                        value);
 /*! <<<```
  *    Sets the peak gain parameter to the given `value` (linear gain) in
  *    `coeffs`.
  *
- *    Default value: `0.f`.
+ *    `value` must be finite and positive.
+ *
+ *    If actually using the bandwidth parameter to control Q,  by the time
+ *    `bw_peak_update_coeffs_ctrl()`, `bw_peak_update_coeffs_audio()`,
+ *    `bw_peak_process1()`, `bw_peak_process()`, or `bw_peak_process_multi()` is
+ *    called, `bw_sqrtf(bw_pow2f(bandwidth) * peak_gain)
+ *    * bw_rcpf(bw_pow2f(bandwidth) - 1.f)` must be in [`1e-6f`, `1e6f`].
+ *
+ *    Default value: `1.f`.
  *
  *    #### bw_peak_set_peak_gain_dB()
  *  ```>>> */
-static inline void bw_peak_set_peak_gain_dB(bw_peak_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_peak_set_peak_gain_dB(
+	bw_peak_coeffs * BW_RESTRICT coeffs,
+	float                        value);
 /*! <<<```
  *    Sets the peak gain parameter to the given `value` (dB) in `coeffs`.
  *
- *    Default value: `-INFINITY`.
+ *    `value` must be finite.
+ *
+ *    If actually using the bandwidth parameter to control Q,  by the time
+ *    `bw_peak_update_coeffs_ctrl()`, `bw_peak_update_coeffs_audio()`,
+ *    `bw_peak_process1()`, `bw_peak_process()`, or `bw_peak_process_multi()` is
+ *    called, `bw_sqrtf(bw_pow2f(bandwidth) * peak_gain)
+ *    * bw_rcpf(bw_pow2f(bandwidth) - 1.f)` must be in [`1e-6f`, `1e6f`].
+ *
+ *    Default value: `0.f`.
  *
  *    #### bw_peak_set_bandiwdth()
  *  ```>>> */
-static inline void bw_peak_set_bandwidth(bw_peak_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_peak_set_bandwidth(
+	bw_peak_coeffs * BW_RESTRICT coeffs,
+	float                        value);
 /*! <<<```
  *    Sets the bandwidth `value` (octaves) in `coeffs`.
  *
- *    Default value: `0.5f`.
+ *    If actually using the bandwidth parameter to control Q,  by the time
+ *    `bw_peak_update_coeffs_ctrl()`, `bw_peak_update_coeffs_audio()`,
+ *    `bw_peak_process1()`, `bw_peak_process()`, or `bw_peak_process_multi()` is
+ *    called, `bw_sqrtf(bw_pow2f(bandwidth) * peak_gain)
+ *    * bw_rcpf(bw_pow2f(bandwidth) - 1.f)` must be in [`1e-6f`, `1e6f`].
+ *
+ *    Valid range: [`1e-6f`, `90.f`].
+ *
+ *    Default value: `2.543106606327224f`.
  *
  *    #### bw_peak_set_use_bandwidth()
  *  ```>>> */
-static inline void bw_peak_set_use_bandwidth(bw_peak_coeffs *BW_RESTRICT coeffs, char value);
+static inline void bw_peak_set_use_bandwidth(
+	bw_peak_coeffs * BW_RESTRICT coeffs,
+	char                         value);
 /*! <<<```
  *    Sets whether the quality factor should be controlled via the bandwidth
  *    parameter (`value` non-`0`) or via the Q parameter (`0`).
  *
  *    Default value: non-`0` (use bandwidth parameter).
+ *
+ *    #### bw_peak_coeffs_is_valid()
+ *  ```>>> */
+static inline char bw_peak_coeffs_is_valid(
+	const bw_peak_coeffs * BW_RESTRICT coeffs);
+/*! <<<```
+ *    Tries to determine whether `coeffs` is valid and returns non-`0` if it
+ *    seems to be the case and `0` if it is certainly not. False positives are
+ *    possible, false negatives are not.
+ *
+ *    `coeffs` must at least point to a readable memory block of size greater
+ *    than or equal to that of `bw_peak_coeffs`.
+ *
+ *    #### bw_peak_state_is_valid()
+ *  ```>>> */
+static inline char bw_peak_state_is_valid(
+	const bw_peak_coeffs * BW_RESTRICT coeffs,
+	const bw_peak_state * BW_RESTRICT  state);
+/*! <<<```
+ *    Tries to determine whether `state` is valid and returns non-`0` if it
+ *    seems to be the case and `0` if it is certainly not. False positives are
+ *    possible, false negatives are not.
+ *
+ *    If `coeffs` is not `NULL` extra cross-checks might be performed (`state`
+ *    is supposed to be associated to `coeffs`).
+ *
+ *    `state` must at least point to a readable memory block of size greater
+ *    than or equal to that of `bw_peak_state`.
  *  }}} */
 
 #ifdef __cplusplus
@@ -230,42 +371,87 @@ static inline void bw_peak_set_use_bandwidth(bw_peak_coeffs *BW_RESTRICT coeffs,
 extern "C" {
 #endif
 
+#ifdef BW_DEBUG_DEEP
+enum bw_peak_coeffs_state {
+	bw_peak_coeffs_state_invalid,
+	bw_peak_coeffs_state_init,
+	bw_peak_coeffs_state_set_sample_rate,
+	bw_peak_coeffs_state_reset_coeffs
+};
+#endif
+
 struct bw_peak_coeffs {
+#ifdef BW_DEBUG_DEEP
+	uint32_t			hash;
+	enum bw_peak_coeffs_state	state;
+	uint32_t			reset_id;
+#endif
+
 	// Sub-components
-	bw_mm2_coeffs	mm2_coeffs;
+	bw_mm2_coeffs			mm2_coeffs;
 
 	// Coefficients
-	float		bw_k;
+	float				bw_k;
 
 	// Parameters
-	float		peak_gain;
-	float		Q;
-	float		bandwidth;
-	char		use_bandwidth;
-	int		param_changed;
+	float				Q;
+	float				peak_gain;
+	float				bandwidth;
+	char				use_bandwidth;
+	int				param_changed;
 };
 
 struct bw_peak_state {
+#ifdef BW_DEBUG_DEEP
+	uint32_t	hash;
+	uint32_t	coeffs_reset_id;
+#endif
+
 	bw_mm2_state	mm2_state;
 };
 
-#define BW_PEAK_PARAM_PEAK_GAIN	1
-#define BW_PEAK_PARAM_Q		(1<<1)
+#define BW_PEAK_PARAM_Q		1
+#define BW_PEAK_PARAM_PEAK_GAIN	(1<<1)
 #define BW_PEAK_PARAM_BANDWIDTH	(1<<2)
 
-static inline void bw_peak_init(bw_peak_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_peak_init(
+		bw_peak_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
 	bw_mm2_init(&coeffs->mm2_coeffs);
-	coeffs->peak_gain = 1.f;
 	coeffs->Q = 0.5f;
+	coeffs->peak_gain = 1.f;
 	coeffs->bandwidth = 2.543106606327224f;
 	coeffs->use_bandwidth = 1;
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->hash = bw_hash_sdbm("bw_peak_coeffs");
+	coeffs->state = bw_peak_coeffs_state_init;
+	coeffs->reset_id = coeffs->hash + 1;
+#endif
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_peak_coeffs_state_init);
 }
 
-static inline void bw_peak_set_sample_rate(bw_peak_coeffs *BW_RESTRICT coeffs, float sample_rate) {
+static inline void bw_peak_set_sample_rate(
+		bw_peak_coeffs * BW_RESTRICT coeffs,
+		float                        sample_rate) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(sample_rate) && sample_rate > 0.f);
+
 	bw_mm2_set_sample_rate(&coeffs->mm2_coeffs, sample_rate);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_peak_coeffs_state_set_sample_rate;
+#endif
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_peak_coeffs_state_set_sample_rate);
 }
 
-static inline void bw_peak_update_mm2_params(bw_peak_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_peak_update_mm2_params(
+		bw_peak_coeffs * BW_RESTRICT coeffs) {
 	if (coeffs->param_changed) {
 		if (coeffs->use_bandwidth) {
 			if (coeffs->param_changed & (BW_PEAK_PARAM_PEAK_GAIN | BW_PEAK_PARAM_BANDWIDTH)) {
@@ -286,51 +472,214 @@ static inline void bw_peak_update_mm2_params(bw_peak_coeffs *BW_RESTRICT coeffs)
 	}
 }
 
-static inline void bw_peak_reset_coeffs(bw_peak_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_peak_reset_coeffs(
+		bw_peak_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_set_sample_rate);
+
 	coeffs->param_changed = ~0;
 	bw_peak_update_mm2_params(coeffs);
 	bw_mm2_reset_coeffs(&coeffs->mm2_coeffs);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_peak_coeffs_state_reset_coeffs;
+	coeffs->reset_id++;
+#endif
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_peak_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_peak_reset_state(const bw_peak_coeffs *BW_RESTRICT coeffs, bw_peak_state *BW_RESTRICT state, float x_0) {
+static inline float bw_peak_reset_state(
+		const bw_peak_coeffs * BW_RESTRICT coeffs,
+		bw_peak_state * BW_RESTRICT        state,
+		float                              x_0) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(bw_is_finite(x_0));
+
+	const float y = x_0;
 	bw_mm2_reset_state(&coeffs->mm2_coeffs, &state->mm2_state, x_0);
+
+#ifdef BW_DEBUG_DEEP
+	state->hash = bw_hash_sdbm("bw_peak_state");
+	state->coeffs_reset_id = coeffs->reset_id;
+#endif
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_peak_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(y));
+
+	return y;
 }
 
-static inline void bw_peak_update_coeffs_ctrl(bw_peak_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_peak_reset_state_multi(
+		const bw_peak_coeffs * BW_RESTRICT              coeffs,
+		bw_peak_state * BW_RESTRICT const * BW_RESTRICT state,
+		const float *                                  x_0,
+		float *                                        y_0,
+		size_t                                         n_channels) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(x_0 != NULL);
+
+	if (y_0 != NULL)
+		for (size_t i = 0; i < n_channels; i++)
+			y_0[i] = bw_peak_reset_state(coeffs, state[i], x_0[i]);
+	else
+		for (size_t i = 0; i < n_channels; i++)
+			bw_peak_reset_state(coeffs, state[i], x_0[i]);
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(y_0 != NULL ? bw_has_only_finite(y_0, n_channels) : 1);
+}
+
+static inline void bw_peak_update_coeffs_ctrl(
+		bw_peak_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(coeffs->use_bandwidth
+		? bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) >= 1e-6f
+			&& bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) <= 1e6f
+		: coeffs->Q >= 1e-6 && coeffs->Q <= 1e6);
+
 	bw_peak_update_mm2_params(coeffs);
 	bw_mm2_update_coeffs_ctrl(&coeffs->mm2_coeffs);
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_peak_update_coeffs_audio(bw_peak_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_peak_update_coeffs_audio(
+		bw_peak_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(coeffs->use_bandwidth
+		? bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) >= 1e-6f
+			&& bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) <= 1e6f
+		: coeffs->Q >= 1e-6 && coeffs->Q <= 1e6);
+
 	bw_mm2_update_coeffs_audio(&coeffs->mm2_coeffs);
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
 }
 
-static inline float bw_peak_process1(const bw_peak_coeffs *BW_RESTRICT coeffs, bw_peak_state *BW_RESTRICT state, float x) {
-	return bw_mm2_process1(&coeffs->mm2_coeffs, &state->mm2_state, x);
+static inline float bw_peak_process1(
+		const bw_peak_coeffs * BW_RESTRICT coeffs,
+		bw_peak_state * BW_RESTRICT        state,
+		float                              x) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(coeffs->use_bandwidth
+		? bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) >= 1e-6f
+			&& bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) <= 1e6f
+		: coeffs->Q >= 1e-6 && coeffs->Q <= 1e6);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_peak_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(x));
+
+	const float y = bw_mm2_process1(&coeffs->mm2_coeffs, &state->mm2_state, x);
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_peak_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(y));
+
+	return y;
 }
 
-static inline void bw_peak_process(bw_peak_coeffs *BW_RESTRICT coeffs, bw_peak_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples) {
+static inline void bw_peak_process(
+		bw_peak_coeffs * BW_RESTRICT coeffs,
+		bw_peak_state * BW_RESTRICT  state,
+		const float *                x,
+		float *                      y,
+		size_t                       n_samples) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(coeffs->use_bandwidth
+		? bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) >= 1e-6f
+			&& bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) <= 1e6f
+		: coeffs->Q >= 1e-6 && coeffs->Q <= 1e6);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_peak_state_is_valid(coeffs, state));
+	BW_ASSERT(x != NULL);
+	BW_ASSERT_DEEP(bw_has_only_finite(x, n_samples));
+	BW_ASSERT(y != NULL);
+
 	bw_peak_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_peak_update_coeffs_audio(coeffs);
 		y[i] = bw_peak_process1(coeffs, state, x[i]);
 	}
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_peak_state_is_valid(coeffs, state));
+	BW_ASSERT_DEEP(bw_has_only_finite(y, n_samples));
 }
 
-static inline void bw_peak_process_multi(bw_peak_coeffs *BW_RESTRICT coeffs, bw_peak_state *BW_RESTRICT const *BW_RESTRICT state, const float * const *x, float * const *y, size_t n_channels, size_t n_samples) {
+static inline void bw_peak_process_multi(
+		bw_peak_coeffs * BW_RESTRICT                    coeffs,
+		bw_peak_state * BW_RESTRICT const * BW_RESTRICT state,
+		const float * const *                           x,
+		float * const *                                 y,
+		size_t                                          n_channels,
+		size_t                                          n_samples) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(coeffs->use_bandwidth
+		? bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) >= 1e-6f
+			&& bw_sqrtf(bw_pow2f(coeffs->bandwidth) * coeffs->peak_gain) * bw_rcpf(bw_pow2f(coeffs->bandwidth) - 1.f) <= 1e6f
+		: coeffs->Q >= 1e-6 && coeffs->Q <= 1e6);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(x != NULL);
+	BW_ASSERT(y != NULL);
+
 	bw_peak_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_peak_update_coeffs_audio(coeffs);
 		for (size_t j = 0; j < n_channels; j++)
 			y[j][i] = bw_peak_process1(coeffs, state[j], x[j][i]);
 	}
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_peak_set_cutoff(bw_peak_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_peak_set_cutoff(
+		bw_peak_coeffs * BW_RESTRICT coeffs,
+		float                        value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value >= 1e-6f && value <= 1e12f);
+
 	bw_mm2_set_cutoff(&coeffs->mm2_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
 }
 
 static inline void bw_peak_set_Q(bw_peak_coeffs *BW_RESTRICT coeffs, float value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value >= 1e-6f && value <= 1e6f);
+
 	if (coeffs->Q != value) {
 		coeffs->Q = value;
 #pragma GCC diagnostic push
@@ -338,9 +687,48 @@ static inline void bw_peak_set_Q(bw_peak_coeffs *BW_RESTRICT coeffs, float value
 		coeffs->param_changed |= BW_PEAK_PARAM_Q;
 #pragma GCC diagnostic pop
 	}
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
 }
 
-static inline void bw_peak_set_peak_gain_lin(bw_peak_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_peak_set_prewarp_at_cutoff(
+		bw_peak_coeffs * BW_RESTRICT coeffs,
+		char                        value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+
+	bw_mm2_set_prewarp_at_cutoff(&coeffs->mm2_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+}
+
+static inline void bw_peak_set_prewarp_freq(
+		bw_peak_coeffs * BW_RESTRICT coeffs,
+		float                       value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value >= 1e-6f && value <= 1e12f);
+
+	bw_mm2_set_prewarp_freq(&coeffs->mm2_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+}
+
+static inline void bw_peak_set_peak_gain_lin(
+		bw_peak_coeffs * BW_RESTRICT coeffs,
+		float                        value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value > 0.f);
+
 	if (coeffs->peak_gain != value) {
 		coeffs->peak_gain = value;
 #pragma GCC diagnostic push
@@ -348,13 +736,34 @@ static inline void bw_peak_set_peak_gain_lin(bw_peak_coeffs *BW_RESTRICT coeffs,
 		coeffs->param_changed |= BW_PEAK_PARAM_PEAK_GAIN;
 #pragma GCC diagnostic pop
 	}
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
 }
 
-static inline void bw_peak_set_peak_gain_dB(bw_peak_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_peak_set_peak_gain_dB(
+		bw_peak_coeffs * BW_RESTRICT coeffs,
+		float                        value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+
 	bw_peak_set_peak_gain_lin(coeffs, bw_dB2linf(value));
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
 }
 
-static inline void bw_peak_set_bandwidth(bw_peak_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_peak_set_bandwidth(
+		bw_peak_coeffs * BW_RESTRICT coeffs,
+		float                        value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value >= 1e-6f && value <= 90.f);
+
 	if (coeffs->bandwidth != value) {
 		coeffs->bandwidth = value;
 #pragma GCC diagnostic push
@@ -362,9 +771,16 @@ static inline void bw_peak_set_bandwidth(bw_peak_coeffs *BW_RESTRICT coeffs, flo
 		coeffs->param_changed |= BW_PEAK_PARAM_BANDWIDTH;
 #pragma GCC diagnostic pop
 	}
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
 }
 
 static inline void bw_peak_set_use_bandwidth(bw_peak_coeffs *BW_RESTRICT coeffs, char value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+
 	if ((coeffs->use_bandwidth && !value) || (!coeffs->use_bandwidth && value)) {
 		coeffs->use_bandwidth = value;
 #pragma GCC diagnostic push
@@ -372,10 +788,59 @@ static inline void bw_peak_set_use_bandwidth(bw_peak_coeffs *BW_RESTRICT coeffs,
 		coeffs->param_changed |= BW_PEAK_PARAM_Q | BW_PEAK_PARAM_BANDWIDTH;
 #pragma GCC diagnostic pop
 	}
+
+	BW_ASSERT_DEEP(bw_peak_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_peak_coeffs_state_init);
+}
+
+static inline char bw_peak_coeffs_is_valid(
+		const bw_peak_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (coeffs->hash != bw_hash_sdbm("bw_peak_coeffs"))
+		return 0;
+	if (coeffs->state < bw_peak_coeffs_state_init || coeffs->state > bw_peak_coeffs_state_reset_coeffs)
+		return 0;
+#endif
+
+	if (!bw_is_finite(coeffs->Q) || coeffs->Q < 1e-6f || coeffs->Q > 1e6f)
+		return 0;
+	if (!bw_is_finite(coeffs->peak_gain) || coeffs->peak_gain <= 0.f)
+		return 0;
+	if (!bw_is_finite(coeffs->bandwidth) || coeffs->bandwidth < 1e-6f || coeffs->bandwidth > 90.f)
+		return 0;
+
+#ifdef BW_DEBUG_DEEP
+	if (coeffs->state >= bw_peak_coeffs_state_reset_coeffs) {
+		if (!bw_is_finite(coeffs->bw_k) || coeffs->bw_k <= 0.f)
+			return 0;
+	}
+#endif
+
+	return bw_mm2_coeffs_is_valid(&coeffs->mm2_coeffs);
+}
+
+static inline char bw_peak_state_is_valid(
+		const bw_peak_coeffs * BW_RESTRICT coeffs,
+		const bw_peak_state * BW_RESTRICT  state) {
+	BW_ASSERT(state != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (state->hash != bw_hash_sdbm("bw_peak_state"))
+		return 0;
+
+	if (coeffs != NULL && coeffs->reset_id != state->coeffs_reset_id)
+		return 0;
+#endif
+
+	(void)coeffs;
+
+	return bw_mm2_state_is_valid(&coeffs->mm2_coeffs, &state->mm2_state);
 }
 
-#undef BW_PEAK_PARAM_PEAK_GAIN
 #undef BW_PEAK_PARAM_Q
+#undef BW_PEAK_PARAM_PEAK_GAIN
 #undef BW_PEAK_PARAM_BANDWIDTH
 
 #ifdef __cplusplus
@@ -395,23 +860,58 @@ class Peak {
 public:
 	Peak();
 
-	void setSampleRate(float sampleRate);
-	void reset(float x_0 = 0.f);
+	void setSampleRate(
+		float sampleRate);
+
+	void reset(
+		float               x0 = 0.f,
+		float * BW_RESTRICT y0 = nullptr);
+
+	void reset(
+		float                                       x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0);
+
+	void reset(
+		const float * x0,
+		float *       y0 = nullptr);
+
+	void reset(
+		std::array<float, N_CHANNELS>               x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0 = nullptr);
+
 	void process(
-		const float * const *x,
-		float * const *y,
-		size_t nSamples);
+		const float * const * x,
+		float * const *       y,
+		size_t                nSamples);
+
 	void process(
 		std::array<const float *, N_CHANNELS> x,
-		std::array<float *, N_CHANNELS> y,
-		size_t nSamples);
+		std::array<float *, N_CHANNELS>       y,
+		size_t                                nSamples);
 
-	void setCutoff(float value);
-	void setQ(float value);
-	void setPeakGainLin(float value);
-	void setPeakGainDB(float value);
-	void setBandwidth(float value);
-	void setUseBandwidth(bool value);
+	void setCutoff(
+		float value);
+
+	void setQ(
+		float value);
+
+	void setPrewarpAtCutoff(
+		bool value);
+
+	void setPrewarpFreq(
+		float value);
+
+	void setPeakGainLin(
+		float value);
+
+	void setPeakGainDB(
+		float value);
+
+	void setBandwidth(
+		float value);
+
+	void setUseBandwidth(
+		bool value);
 /*! <<<...
  *  }
  *  ```
@@ -423,9 +923,9 @@ public:
  * change at any time in future versions. Please, do not use it directly. */
 
 private:
-	bw_peak_coeffs	 coeffs;
-	bw_peak_state	 states[N_CHANNELS];
-	bw_peak_state	*BW_RESTRICT statesP[N_CHANNELS];
+	bw_peak_coeffs			coeffs;
+	bw_peak_state			states[N_CHANNELS];
+	bw_peak_state * BW_RESTRICT	statesP[N_CHANNELS];
 };
 
 template<size_t N_CHANNELS>
@@ -436,60 +936,107 @@ inline Peak<N_CHANNELS>::Peak() {
 }
 
 template<size_t N_CHANNELS>
-inline void Peak<N_CHANNELS>::setSampleRate(float sampleRate) {
+inline void Peak<N_CHANNELS>::setSampleRate(
+		float sampleRate) {
 	bw_peak_set_sample_rate(&coeffs, sampleRate);
 }
 
 template<size_t N_CHANNELS>
-inline void Peak<N_CHANNELS>::reset(float x_0) {
+inline void Peak<N_CHANNELS>::reset(
+		float               x0,
+		float * BW_RESTRICT y0) {
 	bw_peak_reset_coeffs(&coeffs);
-	for (size_t i = 0; i < N_CHANNELS; i++)
-		bw_peak_reset_state(&coeffs, states + i, x_0);
+	if (y0 != nullptr)
+		for (size_t i = 0; i < N_CHANNELS; i++)
+			y0[i] = bw_peak_reset_state(&coeffs, states + i, x0);
+	else
+		for (size_t i = 0; i < N_CHANNELS; i++)
+			bw_peak_reset_state(&coeffs, states + i, x0);
+}
+
+template<size_t N_CHANNELS>
+inline void Peak<N_CHANNELS>::reset(
+		float                                       x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0) {
+	reset(x0, y0 != nullptr ? y0->data() : y0);
+}
+
+template<size_t N_CHANNELS>
+inline void Peak<N_CHANNELS>::reset(
+		const float * x0,
+		float *       y0) {
+	bw_peak_reset_coeffs(&coeffs);
+	bw_peak_reset_state_multi(&coeffs, statesP, x0, y0, N_CHANNELS);
+}
+
+template<size_t N_CHANNELS>
+inline void Peak<N_CHANNELS>::reset(
+		std::array<float, N_CHANNELS>               x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0) {
+	reset(x0.data(), y0 != nullptr ? y0->data() : nullptr);
 }
 
 template<size_t N_CHANNELS>
 inline void Peak<N_CHANNELS>::process(
-		const float * const *x,
-		float * const *y,
-		size_t nSamples) {
+		const float * const * x,
+		float * const *       y,
+		size_t                nSamples) {
 	bw_peak_process_multi(&coeffs, statesP, x, y, N_CHANNELS, nSamples);
 }
 
 template<size_t N_CHANNELS>
 inline void Peak<N_CHANNELS>::process(
 		std::array<const float *, N_CHANNELS> x,
-		std::array<float *, N_CHANNELS> y,
-		size_t nSamples) {
+		std::array<float *, N_CHANNELS>       y,
+		size_t                                nSamples) {
 	process(x.data(), y.data(), nSamples);
 }
 
 template<size_t N_CHANNELS>
-inline void Peak<N_CHANNELS>::setCutoff(float value) {
+inline void Peak<N_CHANNELS>::setCutoff(
+		float value) {
 	bw_peak_set_cutoff(&coeffs, value);
 }
 
 template<size_t N_CHANNELS>
-inline void Peak<N_CHANNELS>::setQ(float value) {
+inline void Peak<N_CHANNELS>::setQ(
+		float value) {
 	bw_peak_set_Q(&coeffs, value);
 }
 
 template<size_t N_CHANNELS>
-inline void Peak<N_CHANNELS>::setPeakGainLin(float value) {
+inline void Peak<N_CHANNELS>::setPrewarpAtCutoff(
+		bool value) {
+	bw_peak_set_prewarp_at_cutoff(&coeffs, value);
+}
+
+template<size_t N_CHANNELS>
+inline void Peak<N_CHANNELS>::setPrewarpFreq(
+		float value) {
+	bw_peak_set_prewarp_freq(&coeffs, value);
+}
+
+template<size_t N_CHANNELS>
+inline void Peak<N_CHANNELS>::setPeakGainLin(
+		float value) {
 	bw_peak_set_peak_gain_lin(&coeffs, value);
 }
 
 template<size_t N_CHANNELS>
-inline void Peak<N_CHANNELS>::setPeakGainDB(float value) {
+inline void Peak<N_CHANNELS>::setPeakGainDB(
+		float value) {
 	bw_peak_set_peak_gain_dB(&coeffs, value);
 }
 
 template<size_t N_CHANNELS>
-inline void Peak<N_CHANNELS>::setBandwidth(float value) {
+inline void Peak<N_CHANNELS>::setBandwidth(
+		float value) {
 	bw_peak_set_bandwidth(&coeffs, value);
 }
 
 template<size_t N_CHANNELS>
-inline void Peak<N_CHANNELS>::setUseBandwidth(bool value) {
+inline void Peak<N_CHANNELS>::setUseBandwidth(
+		bool value) {
 	bw_peak_set_use_bandwidth(&coeffs, value);
 }